Respirator fit testing is performed in the workplace in order to find a respirator that fits the facial features of the wearer and, thereby, ensures a minimum of protection against airborne hazards. The aim of this research is to study the basic mechanisms of air and aerosol penetration through face seal leaks and the filtering materials, and to develop from this knowledge quantitative fit tests that are easy and inexpensive to perform. Two types of respirators in common use will be studied: 1. Disposable respirators which consist mostly or entirely of contoured filtering material. 2. Half and full face piece respirators which have air-purifying cartridges attached to an impermeable body of rubber or silicone. A new size fractionating aerosol generator, developed through prior grant support, will be used to determine the faceseal and filter penetration characteristics of several types of disposable respirators. Tests will be performed on mannequins and human subjects in the breath holding and cyclic breathing modes. From this, a simplified test will be recommended and evaluated for a broad size range of industrial aerosols. For the half and full face piece respirators, a respirator integrity test will be developed and the pressure test will be further simplified and tested. Field studies will be performed to evaluate the new fit tests as predictors for work place protection.